The following relates to toner density control in xerographic applications. It finds particular application in utilizing optical reflection to measure black toner density levels of solid patches placed on a clean belt. Application to other colors and to halftone levels, however, is not excluded.
In copying or printing systems such as a xerographic copier, laser printer or inkjet printer, a common technique for monitoring the quality of prints is to artificially create a test patch of a predetermined desired density. The actual density of the printing material, toner or ink for example, in the test patch can then be optically measured to determine the effectiveness of the printing process in placing this printing material on the print sheet.
In the case of xerographic devices such as a laser printer, the surface that is typically of most interest in determining the density of printing material thereon is the charge retentive surface or photoreceptor on which the electrostatic latent image is formed and subsequently developed by causing toner particles to adhere to areas thereof that are charged in a particular way. In such a case an optical device, often referred to as a densitometer, for determining the density of toner on the test patch is disposed along the path of the photoreceptor directly downstream of the development unit. There is typically a process within the operating system of the printer to periodically create test patches of the desired density at predetermined locations on the photoreceptor by deliberately causing the exposure system thereof to change or discharge as necessary the surface at the location to a predetermined extent.
The test patch is then moved past the developer unit and the toner particles within the developer unit are caused to adhere electrostatically. The denser the toner on the test patch, the darker the test patch will appear in optical testing. The developed test patch is moved past a densitometer disposed along the path of the photoreceptor and the light absorption of the test patch is tested. The density of toner on the patch varies in relationship to the percentage of light absorbed by the test patch.
Xerographic test patches that are used to measure the deposition of toner on paper to measure and control the tone reproduction curve (TRC) are traditionally printed on inter-document zones of photoreceptor belts or drums. Generally, each patch that is printed is a uniform solid halftone or background area. This practice enables the sensor to read values on the TRC.
Many xerographic printing system process controls move physical actuators such as developer bias, charge level and raster output scanner (ROS) intensity to maintain the TRC as measured by an in-line optical sensor. Optical reflective sensing of black solid patches at desirable densities can, in particular, suffer from a limited sensing range. Black toner has minimal diffuse reflection and specular reflection typically saturates near or below the developed mass per unit area (DMA) target level.
To overcome this difficulty, development curve measurement and projection schemes have been implemented (e.g., iGen3 patch generator approach) or high digital area coverage halftones that are somewhat correlated with solids have been used. The latter approach suffers to the extent that the correlation is poor, and the previous approach relies on extrapolation and has hardware overhead costs, software complexity, and bandwidth limitations.
In order to remedy this problem, alternative systems and methods need to be employed to more accurately determine patch density. Such identification can provide more consistent and accurate control over the quantity of toner, i.e. color, utilized for various applications.